The Effect of Basin Scale on Diurnal Streamflow Timing

Hourly streamflow timing, as revealed by diurnal fluctuations in discharge in snowfed watersheds, provides a new tool for understanding transport times and processes in river basins. Travel time delays at different basin scales were measured in nested subbasins (6 to 775 km) of the Tuolumne River in Yosemite National Park throughout the spring 2002 and 2003 melt seasons. The travel time increases with longer percolation times through deeper snowpacks, increases with longer travel times overland and along longer stream channels, and increases with slower in-stream flow velocities. In basins smaller than 30 km, snow properties that determine the travel times through the snowpack dominate streamflow timing. In particular, daily peak flows shift to earlier in the day as the snowpack thins and mean discharge increases. In basins larger than 150 km, snowpack heterogeneity and mixing cause the hour of peak flow to be remarkably consistent, with little or no variation due to snowpack properties. Basins with areas in between 30 and 150 km exhibit different characteristics in different years, illustrating the transition between small and large-scale basin characteristics. Increasing channel travel times as the snowline retreats to higher elevations are not enough to offset the observed decrease in mean snowpack travel times. However, the observed patterns can be reproduced by a model that couples porous-medium flow through evolving snowpacks and free-surface flow in stream channels, provided that the model incorporates snowpack heterogeneity.